Gold is a popular investment for diversifying risk, but its market is subject to speculation and volatility, with its price driven by supply, demand, and shifting sentiment. Historically, it served as a monetary standard, most notably when the Bretton Woods system pegged the US dollar at $35 per troy ounce until the 1971 Nixon shock ended convertibility, and the Swiss franc was the last major currency to abandon gold in 2000. The price is benchmarked by the London gold fixing (since 1919) and traded continuously worldwide under the code "XAU." Unlike most commodities, gold's price is heavily influenced by the massive above-ground stock (estimated at 158,000 tonnes in 2006) and changes in investor sentiment, rather than annual mine production (about 2,500 tonnes). Central banks and the IMF hold roughly 19% of all above-ground gold, and the Washington Agreement on Gold (first signed in 1999, extended in 2009 and 2014) limited their sales, while countries like Russia and China have signaled interest in increasing their reserves.